Errrrm.. Yeah. Slooms.

So, before adding Slooms you might need to nerf sleep a little. (or just leave it as it is because DMs are supposed to be hard)
Just retake the style of the Shrine of Slumber levels (were they like the aurora isles?), make Slooms spawn more, make super hard battle rooms, and at the end of the mission just make a giant Sloombargo that has more attacks than sleep mist and whatever the Slooms have.

That, or just use your imagination because I'm not a descriptive person.

EDIT: And maybe make Zee Cores a rarer drop, or actually have them used in a crafting recipe.